Apparatus for vacuum casting of uranium



Feb. 26, 1957 Filed Dec. 8, 1944 H. A. WILHELM EI'AL APPARATUS FORVACUUM CASTING `0F' URANIUM 2 'sheets-sheet 1 Feb. 26, 1957 H. A.WILHELM ETAL 2,782,475

APPARATUS FOR VACUUM CASTING oF URANIUM Filed Deo. 8, 1944 2sheets-sheet 2 APPARATUS FOR VACUUM CASTING OF URANIUM Harley A. Wilhelmand C. I". Gray, Ames, low-a, assignors to the United States of Americaas represented by the United States Atomic Energy Commission ApplicationDecember 3, 1944, Serial No. 567,283

2 Claims. (Cl. 22-73) The invention relates to a process and apparatusfor treating uranium metal.

Where uranium has been obtained in the form of substantially pure metal,the reactivity of the uranium causes impurities in the form of uraniumcompounds, such as uranium oxide, to be associated with the metalimmediately upon exposure of the metal to ordinary atmosphericconditions. The formation of uranium oxide, for example, givestheuranium metal a hard, dense skin or tilm which tends to increase inthickness as.the metal is continued to be exposed to air.

Uranium metal in the form of an ingot or biscuit is produced by thereaction of a uranium halide such as Ulit and a reducing metal such ascalcium or magnesium at a temperature sufficiently high to form moltenuranium and a molten slag and under conditions such that the uraniumremains in the liquid state for atime suliicient to permit layerseparation of a molten uranium and a slag layer. The resulting uraniumupon cooling is relatively pure but may also include impurities such` asuranium carbide, calcium oxide, or the like, that are solid at themelting and casting temperature of uranium. Such impurities are usuallyon the surface of the ingot and are associated with the skin of uraniumoxide.

lt is found that uranium metal may be separated from the solidimpurities such as the skin portion of the ingot by heating a uraniumVmass containing such impurities to at least the melting point of theuranium metal and then withdrawing the molten uranium away from thesolid impurities. Thus it has been found that the skin portion of auranimn body has a higher melting point than the interior thereof andthat it is possible to heat the body at a temperature such that theinterior of the body becomes molten while the exterior skin remainsessentially solid. The separation may be carried outV by supporting theuranium mass in a melting chamber over a closed opening or aperture inthe bottom of the chamber. After the uranium metal mass has becomemolten the aperture is opened, thereby Causing the molten uranium toilow through the aperture and away from the solid impurities. Where thesolid impurities are in the form of a skin around the uranium metalmass, the skin may be punctured as the aperture is opened and the moltenuranium released from the interior of the mass.

In the reaction in which calcium, magnesium, orA other reducing metal isreacted witha uraniumV halide, a portion ofthe reducing metal may beassociated with the uranium metal mass. Such reducing metal, andparticularly magnesium, is volatileV atthemelting and castingtemperatures of uranium and may be removed for the most part during themelting of the uranium ingot. This invention relates to improvements inthe apparatus disclosed in the copending applicationY of'Frank` I-I.Spedding et al., Serial No. 554,920, tiled September 20, 1944,` and alsonto improvements in the processes disclosed inthe copending applicationof George Meister, Serial No. 533,112, led April 28, 1944, now PatentNo.2,756,138, granted Iulyf2`4, 1956. y

States aten-t It is an object of the invention to provide a process andapparatus for purifying uranium metal; for separating uranium metal fromvarious impurities associated vwith it that are in solid form at thecasting temperature of uranium; and for removing from the uraniumimpurities that are volatile 'at the melting and casting temperature ofuranium. A further object of the invention is the provision of a processand apparatus for casting molten uranium metal into a carbon moldwithout excessive formation of uranium carbide. It is a still furtherobject of the invention to obtain a shaped product of exceptionally pureuranium metal by casting molten uranium metal into a carbon mold. Otherobjects and advantages will appear from the following description anddrawings appended thereto.

A In the drawings, in which like reference characters refer to likeparts: t

Fig. l is a sectional elevation of one embodiment of the invention;

Fig. 2 is a sectional elevation of a modified form of the invention;

Fig. 3 is a sectional View taken on line 3-3 of Fig. l;

Fig. 4 is a sectional view taken on line 4-4 of Fig. 1;

Fig. 5 is a sectional view taken on line 5-5 of Fig. 2; and

Fig. 6 is a sectional view of a modified form of the casting chambershown in Fig. 5.

.in one embodiment of the invention, an apparatus 1 is provided as shownin Fig. 2 which includes a crucible 3 having a melting chamber 2 formedwithin and adapted to contain one or more uranium ingots or biscuits, 4.The Crucible is preferably of cylindrical form and comprises cruciblewalls 6 and sloping bottom 7. A circular lid E is adapted to engage theupper end of the Crucible. Preferably, the lid has an opening 9 whichfacilitates the escape of volatile impurities vaporized during themelting and casting operation. To prevent direct impingement of thevolatile irnpurties upon outer parts of the apparatus, cap 11 isprovided which includes sideopenings 12 to permit the escape ofvaporized impurities from the cap. If it is desired to enlarge meltingchamber 2, suitable extensions may be placed on the upper end ofcrucible walls 6 with the lid 8 engaging the topof the uppermostextension.

The crucible 3 has an aperture 16 extending vertically through thebottom of the Crucible, the aperture being sufficiently large for thepassage of molten uranium therethrough. Although the size of aperture 16isso'mewhat dependent upon the character of the solid skin irnpurities,it has been found that circular apertures of a diameter ranging from 1A;inch to two inches are'suitable, for this purpose. The aperture 16 maybe closed by suitable means such as a plug 18 which seats on vslopingbottom 7 or, preferably, into rabbrets providedV for thatV purpose. Arecess 52 is provided in the lower portion of the Crucible 3 to receivetemperature measuring means used during the process. '1

The Crucible 3 rests upon and is supported by a mold 13 which includesone or more casting chambers 1,4. Chambers 14 are of the shape of thedesired casting and should be of a size sucient to receive all'of theuranium metal contained in the ingots 4. An opening or bore 49A isVformed in mold 13 to permit the insertion of a temperature-measuringdevice such as 4a thermocouple. An

opening or bore 32 extends vertically through the center of mold 13. Theupper edgesof the interior portion of mold 13 are preferably bevelled todirect the flow of molten uranium into casting chambers 14 from aperture16'. If desired, one-or more casting chambers 14 may include a lower andsmaller castingV chamber 15l as shown in Fig. l. In this way uraniumcastings of different-sizes*- may. be-producedin the onecastingloperation; One for more vents 19 are formed in the upper wall ofthe mold to facilitate evacuation of the casting chambers.

The elements of the apparatus above described may be formed of anyrefractory material suitable for the treatment of uranium. Beryllia,thoria, alundum, magnesia, sillimanite, or the like may be used althoughthere is a tendency for such refractory materials to contaminate thecast uranium. Preferably, all of the elements of the apapratus in directcontact with the uranium are formed of a hard, dense form of carbon suchas graphite that may be machined to the proper size and shape. Althoughthere is a tendency for the molten uranium to react with the carbon toform uranium carbide, for most purposes such carbide formation ispreferred to contamination resulting from the use of other refractorymaterials for the apparatus; and the uranium carbide formation may beminimized by proper casting procedure.

The mold 13 rests upon block 21, which may be formed of a material suchas rebrick, a metal or graphite, containing channels 22 and 23 for thepurpose of evacuating the system or for the introduction of an inertatmosphere. This block is supported by a base 24 which is preferably awater-cooled brass casting including a water chamber 26 and inlet andoutlet connections 27. Channel 28 extends through base 24 in alignmentwith channel 23 of the block 21.

To dislodge plug 18, a trip rod 29 is inserted in openings provided inthe block 21 and mold 13 for that purpose. Trip rod 29 is a two pieceunit; the upper part 55, preferably formed of graphite, engages theunder surface of the plug and is a-dapted to lift the plug free frombottom 7 of the crucible for a sufficient distance to permit the moltenuranium to tlow beneath the plug and through aperture 16. The lowersection 60 of trip rod 29 is made of metal preferably of steel and itengages the upper section 55 at point X within the provided opening inthe mold. The diameter of this provided opening in the mold is such withrespect to the sizes of the two sections of the trip rod that the uppersection rests freely but firmly and in alignment on the lower section.

Where the system is operated in vacuo, a T-itting 33 having a leg 34eonnectible with vacuum and an end opening 36 is attached ot the exitend of channel 28. A sleeve 37 is inserted into opening 36 to receiverod 29. A resilient seal 38 of rubber or the like is placed over thesleeve and rod to prevent the access of air to the system. The interiorof seal 33 may be lubricated to secure a tight engagement of the seal tothe rod without unduly restricting the movement of the rod. Suitableevacuating means (not shown) is attached to vacuum connection 34.

To minimize heat loss from melting chamber 2 and upper portion ofcasting chambers 14, insulation 39 may be provided around the mold andcrucible. Preferably, the insulation is in the form of curved sheetsfabricated from thermal-setting sillimanite yand lsawdust which isshaped and then red at a high temperature. The insulation is laid uparound the apparatus, resting upon water cooled base 24, and `a flatcircular sheet 40 of a similar material is placed over the top, sheet 40being protected by cap 11 from the deleterious effects of the volatileimpuritiles vaporized in the melting chamber.

As the casting operation is preferably performed in a vacuum, a shield41 is used to envelop the apparatus. A water cooled extension 42,resting on ilanges of the base 24, supportsythe shield. The extension42, of brass or the like, includes water chambers 43 and inlet andoutlet connections 44. An airtight seal is provided by rubber gaskets 46interposed between the lower end of the shield and the extension 42 andalso between the extension and the base 24. Sealing material 47, whichmay be a high temperature melting point wax, pitch or the like may be byan induction coil 4S which is ararnged to surround shield 41 and to beraised and lowered vertically into any desired position with respect tothe apparatus. To permit the use of temperature measuring means such asthermocouples or the like, an opening 49 is provided through waterchamber 26 of the base, block 21 and the wall of mold 13. A thermocouple51 is inserted in the opening 49 with its upper end resting in recess 52of the crucible, thus coming into close proximity with the meltingchamber 3. The thermocouple may be sealed to open- 49 by means ofsealing material.

The base 24 is permanently mounted on a cart (not shown) and thethermocouple S1, T-tting 33, sleeve 37, seal 3S, steel portion 60 of rod29, water hose (not shown) on connections 27, block 21 and water cooledextension 4Z are more or less permanently mounted in position as shown.This assembly is used for a number of runs without dismantling. Onlyoccasional repair or cleaning makes it necessary to remove any of theseparts from the set-up. With these parts in position the apparatus isthen assembled for each run by placing mold 13 on block 21 with opening49 in alignment. The graphite section 55 of rod 29 is put in place andcrucible 3 is positioned on mold 13 with recess 52 directly over opening49. Plug 18 is seated over orice 16 and the uranium ingots or biscuits 4are placed within melting chamber 2. It has been found that in theevacuation of the apparatus there is a tendency for the plug 13 tobecc-me unseated with the result that orifice 16 is not completelyclosed when the ingots are being melted. To prevent this, it isdesirable to place `a small amount of a finely divided uranium compound,such as uranium oxide 53, over the plug followed by resilient turnings54 of uranium metal. The weight of the uranium ingots upon the turningscauses the plug to securely engage the bottom of the crucible. Afterassembling the crucible, insulation 39 and sheet 4t) are laid up aroundthe apparatus and shield 41 is positioned over the insulation.Evacuating means is then attached to conection 34. Induction coil 48 islowered around the shield 41 until the bottom end of the coil isapproximately one inch below the bottom of crucible 3.

In carrying out the process, the induction coil is operated until thetemperature within recess 52 reaches l0O0 C., this temperature beingmaintained for about eight to twelve minutes. The temperature of theuranium is then raised to approximately 1360 C. and this temperaturemaintained for approximately l() minutes. The uranium is then cooled toapproximately l200 C., and plug 18 is unseated by trip rod 29 therebypermitting the molten uranium to ow through aperture 16 and into castingchambers 14. The coil 48 is then lowered approximately 3 inches andoperated for ten minutes. The current is then shut off, and theapparatus is allowed to cool. After the cooling, the Vacuum is releasedand the apparatus disassembled for the removal of cast uranium.

1n melting the uranium in chamber 2, it is preferred that the heating becarried out at such a rate that the temperature of the uraniumthroughout the ingots does not apprecably lag behind the temperature ofthe uranium adjacent to the walls of crucible 3. Where an induction coilis used to heat the crucible, the temperature of the uranium in thecrucible is controlled by increasing or decreasing the power supply tothe induction coil. In this way, uranium carbide formation in themelting chamber is lessened and the violent vaporization of impuritiesis prevented. Such heating is particularly desirable in the initialstage of melting the uranium. A slight lag between the temperature ofthe inner and outer portions of the ingot is not objectionable, althoughthe temperature of the outer portion should not be permitted to rise toa point at which there is excessive formation of uranium carbide.

As there is a tendency for uranium metal to react with air to` form anexterior layer or coating of dense4 uranium oxide it mayv be necessaryto push plug 1 8 upwardly into thev ingot when tapping the moltenuranium. In this way, the outer skin of impurities on the ingot may bebroken or punctured and the molten uranium from insidethe skin is;allowed to ilow through aperture 16.

Preferably, the inner surface of melting chamber 2 and particularly thesurfaceof sloping bottom 7 have adherable characteristics which aid inretaining the solid impurities in chamber 2. It has been found that suchimpurities tend to adhere to machined graphite as the molten uraniumflows through aperture 16. Where a plurality of ingots are treated, theinterior surface of walls 6 retain theportions ofthe skin of the topingots.

Pipes or hollows in the upper surface of the casting may be preventedand better castings are generally obtained by regulatingv the cooling`of the cast metal in such a manner that the solidication begins at thebottom ofthe castingY chamber and proceeds upwardly. This may be doneeither `by having the upper portion of mold 13 =at a higher temperaturethan the lower portion when the casting is made, or by gradually raisingcoil 48, while still in operation, after the uranium has flowed intochambers 14. Preferably, moderate heat is applied to the upper portionof the casting chamber while the cast uranium is cooling.

In a specic example of a casting operation using the apparatusdescribed, the lower half of the 25-inch induction coil 48, having thebottom of the coil one inch below the top of mold 13, was operated at 35kilowatts until 1300 C. was recorded by the thermocouple 51. The entirecoilY was then operated atV 35 kilowatts for three to eight minutes. Thelower half of the coil was then operated at 35 kilowatts to obtain atemperature of 1310 C. which was held for seven minutes. The coil wascut. oft and when the uranium had cooled to l260 C., itwas castimmediately. The coil was then lowered approximately three inches andoperated at 25 kilowatts for five minutes.

ln afurther specific example of the process, the lower half of the coilwas operated at 30 kilowatts with the bottom of the coil one inch belowthe top of mold 13 until 13009 C. was recorded'by the thermocouple. Theentire coil was then operated, at 35 kilowatts for three to eightminutes. The upper halfof the coil was then shorted out and the lowerhalf` of the coil was operated at 35 kilowatts until a-temperature ofl300 C. was obtained. The uranium.wasv cooled to l260 C. and pouredimmediately. VThe coil was then lowered three inches and the-lower halfwa-s heated at 20 kilowatts for tiveV minutes.

In heating.- uranium ingotsV resulting from the reaction of a uraniumhalide and a substantial excess of a reducing metalY suchas. calcium` ormagnesium, there is a rapid vaporization of the unreacted reducing metalbefore the molten uranium. is tapped. This sudden vaporization, at timesresembling an explosion, takes place at approximately l050 C. wheremagnesium is used, and between 1300 C. and 1400 C. Where calcium isused. It has been found that such violent vaporization may be preventedby using an excess of reducing metal just sufficient to obtain acomplete reduction of the uranium halide.

In =a modification of the apparatus as shown in Fig. 1, elements similarto those described in connection with Fig. 2, have the same referencenumerals as in Fig. Z. In the modified construction, the Crucible 3 andmold 13 are separated by a channel plate 56 containing channel 57leading from orifice 16 to casting chamber 1d. As shown in Fig. 4,channel 57 may widen directly under orifice 16 to prevent restriction ofthe ilow of molten uranium through the channel. The vertical portion 58of the channel directs the stream of molten uranium downward into thecenter of casting chamber 14. A

ventSS!v facilitates evacuation of chamber 14 without dis-Y turbing plug10,. The trip rod 29 is of three sections of rod, and one oifset arm 62.The two lower sections 63 and 64 of rod up to point X and the oiset arm62 are of steel while the section 55 of rod above X and to the valveismade of graphite. The graphite collar 61 gives a close but sliding fitaround the graphite section of the push rod. Such a fit does not allowmolten metal to leak down into the bore 32 in the wall of the mold. Thisembodiment of the invention is particularly suitable for a singlechamber mold as shown in Figs. l and 3. The single casting chamber 14 ispreferably olf center to provide greater wall thickness for the bore oropeningV 32 containing trip rod 29. lf desired, a smaller castingchamber 15 may extend from the lower portion of mold 14. If desired astrut extending across -chamber 2S (not shown) may be provided to centerthe trip rod. Since the casting chamber 14 extends over the center ofthe mold, the lower portion of trip rod 29 may be centered by sleeve 36and be connected to the upper portion by a horizontal rod 62 in channel23. The upper portion 5S of rod 29 extends to plug 18 through a bore 32in the side wall of the mold. Channel 23 is enlarged to accommodate rod62.

. The volatile -impurities are removed for the most part by heating theuranium ingot in the melting chamber at a temperature of between 1200 C.and 1500 C. While temperatures higher than 1500 C. may be used forvaporizing the impurities, it is preferred that the temperature be keptbelow 1500 C. to minimize the formation of uranium carbide. Invaporizing the vaporizable impurities, the temperature should be keptelevated for suicient time to permit the vaporizable impurities to beremoved from the uranium. The length of -tirne for such vaporizationshould be as short as possible to minimize the formation of uraniumcarbide which latter increases as the time of Contact between moltenuranium and carbon is increased.

To minimize the formation of uranium carbide in casting chamber 14, itis preferred that the molten uranium in the melting chamber 3 be cooledbefore plug 18 is unseated and the molten uranium is permitted to Vflowthrough aperture 16 into the casting chamber. Better Castings areobtained and the formation of uranium carbide lessened where the moltenuranium in 4the melting chamber is cooled to slightly above its meltingpoint before it is cast. Temperatures between ll00 C. and l300 C. havebeen found suitable for casting the molten uranium without causing asubstantial formation of uraniumrcarbide in the casting chamber. Thecooling step is particularly important in connection with the step ofvaporizing the vaporizable impurities, because the temperature of themolten uranium is then maintained in the melting chamber at atemperature of between l200 C..and 1500" C.

Hitherto, carbon receptacles could not be used for making substantiallypure castings of uranium metal inasmuch as the melting point of pureuranium had previously been considered to be approximately 1800 C.Consequently, in casting such uranium, the metal was raised to 1860 C.before pouring it into the carbon mold with the result that a largeamount of uranium carbide was formed on the exterior surface of theuranium and then dissolved in the liquid metal at such temperature. Thecarbide formation of the cast uranium was also increased by additionaltime required for the molten uranium poured at a temperature of 1800a C.to be cooled to a point at which no uranium carbide was for-med. Afurther disadvantage of this process was that the carbon molds wereseverely attacked by the molten uranium and soon had to be replaced orrelined. The resulting metal by this treatment was high in carboncontent.

Fol-lowing the present invention molten uranium can be melted ingraphite crucibles and can then be cast into carbon molds with little orno carbide formation, this resulting from the discovery that the truemelting point of uranium is not approximately l800 C. but rather, is inthe neighborhood of 1100 C. At this lower temperature there is littletendency for the molten uranium to combine with the interior surface ofthe melting crucible and the final carbon'mold to form uranium carbide.The greatly decreased time of cooling the uranium from the temperatureat which it is cast to the temperature at which no uranium carbide isformed also contributes to obtaining cast uranium metal exceptionallyfree from uranium carbide. Where pure molten uranium was placed -incontact with carbon at varying temperatures, it was found that -theamount of carbon that combined with the uranium increased rapidly withthe temperature, as shown in the `following table:

In this respect, the requirements that the uranium be maintained at anelevated temperature to remove the vaporizable impurities and theuranium be in contact with the carbon mold for a minimum of time and ata lower temperature, are seemingly incompatible. However, the castingprocess of the present invention, by maintaining the uranium at a highertemperature in the melting chamber until the impurities are vaporized,in spite of carbide formation, and then pouring into the casting chamberat a lower temperature With relatively fast solidication, satisfies bothrequirements, as the major por-tion of the uranium carbide formed in themelting chamber remains in the melting chamber as part of the retainedsolid impurities.

ln addition to providing cast uranium substantially free from uraniumcarbide, the process has the further advantage of increasing the life ofthe carbon molds. Substantial formation of uranium carbide causes apitting ot the interior surface of the mold thereby necessitatingmachining of the mold to provide a smooth and regular surface for thenext casting. In many cases the mold will be of such irregular shapethat the machining can be carried out only with diliiculty if at all.Where the mold is machined down to provide a smooth interior surface,the Wall thickness of the mold is considerably decreased and the castingchamber is enlarged thereby preventing uniform castings. Also,substantial carbide formation tends to cause the casting to adhere tothe mold with the result that there is a high probability of breakage ofthe mold in removing the casting.

The terms uranium ingot and uranium biscuit, as used in the descriptionand claims, include any metallic uranium mass containing impuritiesinternally and/or on the surface of thevmass. Thus, it. has been used todefine a crude uranium ingot or biscuit which contains impuritiesresulting from the production of the uranium, as well as any mass ofuranium metal which has been exposed to air, thereby having a skin orlayer of uranium oxide on the outer surface of the mass.

The above detailed description of preferred apparatus and processes isfor the purpose of illustration, and the invention is to be limited onlyby the scope of the following claims.

We claim:

1. An apparatus for treating uranium in vacuo which comprises a heatresistant base, a refractory block supported on said base, a moldsupported on said block and having a casting chamber therein, a cruciblesupported on said mold and having a melting chamber therein, saidchambers connected by an aperture, means adapted to close and open saidaperture, insulation around said mold and crucible, a Shield over saidinsulation and in cngagernent with said base, whereby the assembly maybe hermetically sealed, and an induction heating coil surrounding saidapparatus along part of its height and being adjustable along itsvertical axis, the uranium-contacting surfaces of said crucible, saidmold and said clos* ing means consisting of graphite.

2. An apparatus for treating uranium, which comprises a Crucible havinga melting chamber therein, a perforate lid on said crucible, a cap overthe perforate portion of the lid, the sidewalls of said cap having atleast one opening therein, a mold below and contiguous with saidchamber, said mold and melting chamber being connected by an aperture inthe lower part of said chamber, means for closing said aperture, a shellenclosing the Crucible and mold-assembly, means for establishing asubatmospheric pressure within said shell, and heating means extendingalong part of the height of the'assembly arranged adjustably along thevertical axis of said apparatus, the uranium-contacting surfaces of saidcrucible, said mold and said closing means consisting of graphite.

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1. AN APPARATUS FOR TREATING URANIUM IN VACUO WHICH COMPRISES A HEAT RESISTANT BASE, A REFRACTORY BLOCK SUPPORTED ON SAID BASE, A MOLD SUPPORTED ON SAID BLOCK AND HAVING A CASTING CHAMBER THEREIN, A CRUCIBLE SUPPORTED ON SAID MOLD AND HAVING A MELTING CHAMBER THEREIN, SAID CHAMBERS CONNECTED BY AN APERTURE, MEANS ADAPTED TO CLOSE AND OPEN SAID APERTURE, INSULATION AROUND SAID MOLD AND CRUCIBLE, A SHIELD OVER SAID INSULATION AND IN ENGAGEMENT WITH SAID BASE, WHEREBY THE ASSEMBLY MAY BE HERMETICALLY SEALED, AND AN INDUCTION HEATING COIL SURROUNDING SAID APPARATUS ALONG PART OF ITS HEIGHT AND BEING ADJUSTABLE ALONG ITS VERTICAL AXIS, THE URANIUM-CON- 